House extension cost per m²

Less.co.uk, updated in March 2026, gives the most detailed breakdown of house extension cost per m² by type. Its figures are Midlands averages and exclude VAT and professional fees, so they work best as a starting point that you then adjust for your region and specification. Less says a budget specification starts from £1,200 per m² and that premium work can exceed £3,500 per m².

The pattern in the table is worth understanding. Side returns cost more per square metre than a plain rear extension because they are usually narrow, close to boundaries and often involve party walls, while double storey and over garage extensions come in lower per square metre because the foundations and roof are spread across more floor area or are already partly in place.

Building costs are higher in London and the South East than in most of the country, partly because labour costs more. Less publishes a regional table for a mid range single storey extension, which excludes VAT, professional fees and interior fit out. It treats the Midlands as the average and shows the South East 17% higher.

Other sources give their own uplifts. The FMB says London and the South East run '30-50%' more per square metre than the national average for single storey work, Homebuilding and Renovating adds 20 to 25% for London and the South East on side returns, and MyJobQuote puts two storey work in the South East 10% to 25% above the UK average. None of them gives prices for Sussex in particular, so treat the South East figures as a broad guide and use local quotes to see where your project sits.

To use a rate per square metre, multiply it by the floor area the extension adds. For a two storey extension, add the floor area of both storeys together. Less gives worked examples at its mid range Midlands rate of £2,000 per m², excluding VAT and professional fees, which show how quickly the total climbs with size. Remember that these are examples at a Midlands rate, and Less's own South East range for single storey work is higher.

• Bi-fold doors: £3,000 to £6,000 (Less.co.uk)
• Roof lantern: £1,500 to £3,500 (Less.co.uk)
• Underfloor heating: £50 to £80 per m² (Less.co.uk)
• Fitted kitchen: £8,000 to £20,000 or more (Less.co.uk)
• Difficult ground: adds £2,000 to £8,000 (Less.co.uk)
• Restricted access: adds 5 to 10% (Less.co.uk)
• Professional fees: £3,000 to £10,000 (Less.co.uk)

Ask each builder whether their rate is based on internal floor area or the external footprint, because the difference can be noticeable on a small extension. Then add the items that sit outside the rate. Less lists these extras:

Dividing each quote by the floor area is a useful sense check, but only if the quotes cover the same scope. One builder's rate might include the kitchen, flooring and decoration while another's stops at plastered walls, which makes the second look cheaper when it is not. Ask for itemised quotes based on the same drawings, with VAT, provisional sums and exclusions stated clearly, and check who is responsible for building control and party wall notices.

Timing matters to the overall cost as well. MyJobQuote gives 9 to 12 weeks for a single storey side return, 10 to 14 weeks for a wraparound or kitchen extension and up to 14 weeks for a two storey extension, all before design and approvals. For a quick estimate using Less's rates, try our extension cost calculator, and to see what builders near you would charge, fill in our short form or call 01273 115520 for up to 3 free quotes from checked local builders, with no obligation. You can also read our single storey extension cost guide for more detail.